Packaging structures of pressure sensor chips, including electrical connections and protective structures, are proposed in U.S. Pat. No. 6,393,922 and Japanese Patent Application No. 10-153508. In U.S. Pat. No. 6,393,922, the pressure sensor chip and bonding wires are sealed with a filling material. In Japanese Patent Application No. 10-153508, an aluminum pad formed on a semiconductor substrate is coated with a Ti/Pd film, and the semiconductor substrate, thereby constituting a diaphragm, is covered with a protecting film and a silicone gel. The protecting film is made of silicone oxide, silicon nitride or the like.
If the pressure sensor chips are exposed to corrosives, such as exhaust gases from engines, the above structures will not provide sufficient protection. Moreover, the silicone gel is provided over a gage formed in the diaphragm. The silicone gel reduces the sensitivity of the pressure sensor, and its internal stress results in excessive stress to the diaphragm and variations in the sensitivity characteristic.